Microphones used in handsfree telephone apparatus commonly utilize an omnidirectional microphone capsule in an assembly which, when installed in the apparatus, provides a relatively omnidirectional response characteristic. Such a microphone assembly is readily integrated into the apparatus since only a single small opening through a housing of the apparatus will provide effective acoustic coupling from the exterior of the housing to the capsule installed therein. These microphones, however, are invariably afflicted with an unnatural "barrel" or hollow sound resulting from acoustic reflections and room reverberation as well as a bothersome background noise sensitivity common to most handsfree telephone apparatus.
The more common problems of acoustic and mechanical coupling between the microphone of a handsfree telephone apparatus and its immediate environment are described briefly in U.S. Pat. No. 4,528,426 Fatovic et al which issued July 9, 1985. A solution is disclosed therein whereby adverse effects from the reflecting surfaces of the apparatus housing enclosing the microphone are minimized so that desired directional properties of the microphone may be retained. According to Fatovic et al, the microphone is isolated from the housing by being mounted within an acoustic foam covering. This minimizes the effects of the microphone housing on the directional characteristics of the microphone capsule, whereas mechanical vibration damping is provided by the resilient nature of the foam covering.
It has been determined, however, that the use of foam as an isolating material may not resolve the difficulties of the prior art and under certain circumstances may even exacerbate existing problems and add new ones. For example, if the resonant frequency of the microphone capsule/foam assembly is too high, meaning that it is within the range of from 300 Hz to 3 kHz, hereinafter referred to as the "voice band", then the vibration isolation performance is actually worse than it would be in the absence of any resilient suspension.
Other problems attributable to the use of foam for mounting a directional microphone within a handsfree telephone apparatus relate to quality control In this regard, it has been determined that manufacturing tolerance variations in the foam result in a substantial performance variation, notably in the directional characteristics of the microphone assembly Moreover, it has been found that the physical characteristics of the foam do not lend themselves well to high-volume, quality manufacturing in both manual and robotic environments This latter consideration bears, of course, directly on the cost effectiveness of manufacturing handsfree telephone apparatus, the achievement of which is desirable and necessary in either of such manufacturing environments.